High strength gamma-prime-strengthened nickel based superalloys used in gas turbine hot sections are generally described as those which have high strength even at temperatures of 1600 degrees Fahrenheit or higher. Generally, in order to restore both dimension and geometry of damaged hot section components such as turbine blades and nozzle guide vanes and maintain elevated temperature properties (both environmental resistant and creep resistant properties etc), advanced superalloy filler materials are laser deposited onto worn or damaged areas of components. Thus, damaged advanced turbine superalloy (including directionally solidified and single crystal cast) components are completely repaired and restored to their original geometry and dimension with excellent elevated temperature properties.
Advanced nickel-based superalloys with high volume fraction of gamma prime precipitates, (i.e. gamma-prime-strengthened nickel based superalloys) such as INCONEL® 738 manufactured by Special Metals Company, René® 80, manufactured by Reade Advanced Materials, and CM-247 manufactured by The C-M Group, as well as single crystal or directionally solidified (DS) superalloys, are susceptible to cracking in both the heat affected zone and in weld beads resulting from welding using filler alloy having the same or similar composition (i.e. “even-matched”) as the base metal (i.e. the substrate). Therefore, many weld repair processes are carried out using conventional solid solution strengthened welding alloys such as INCONEL® 625 manufactured by Special Metals Company, or oxidation resistant high strength Co based weld fillers, which are softer (i.e. “under-matched”) than most gamma-prime-strengthened nickel based superalloys, due to the low Al and Ti content. However, the use of under-matched filler is not desirable for component repairs that require considerable strength. Further, the large thermal expansion mismatch between the high strength Co base fillers and the nickel superalloy substrate could produce high thermo-mechanically induced damage and low thermal mechanical fatigue life during service. To extend the repaired component life and to repair the high-stressed region, ideally the weld metal used should have either the same or close composition as the base metal so that the thermal expansion and creep properties of the weld will closely match the base metal. This is particularly attractive for advanced nickel based superalloy components. However, it is very difficult to produce a crack-free weld with nickel based alloy filler with high Al and Ti contents due to segregation as well as solidification shrinkage or thermal contraction or shrinkage strains from gamma prime precipitation.
Several approaches have been taken toward eliminating the cracks in nickel based superalloy weld buildup for component repairs that require considerable strength. U.S. Pat. No. 4,336,312 describes a combination of a controlled chemical modification of a cast nickel-based superalloy material along with a pre-weld thermal conditioning cycle. U.S. Pat. No. 6,364,971 describes a laser welding technique used following a pre-conditioning hot isostatic process. U.S. Pat. No. 633,484 describes a welding technique wherein the entire weld area is preheated to a maximum ductility temperature range, and this elevated temperature is maintained during the weld and solidification of the weld. These patents are incorporated herein by reference.
Other approaches include 1) laser powder weld build-up while heating the component substrate (see U.S. Pat. Nos. 5,106,010, 6,037,563, 6,024,792, and EP patent 0861927); and using low energy laser beams to re-heat each deposited layer (see U.S. Pat. Nos. 5,900,170, 5,914,059, and 6,103,402). There patents are incorporated herein by reference. While heating the components during laser powder welding is effective, it is an expensive process, can cause distortion of the component, and has the potential of affecting the microstructure of the superalloy due to incipient melting at the grain boundaries. For example, turbine blades are often repaired using a technique known as hot-box welding. Hot-box weld repairs may take eight hours or more to complete, and the requirement for working inside of the hot box to maintain the elevated temperature makes it difficult to perform such welds robotically.
Techniques have therefore been developed that permit weld build-up at room temperature for component repairs that require considerable strength. US patent application 20080210347, incorporated herein by reference, discloses a method for welding superalloy components at ambient temperature conditions while reducing the propensity of the superalloy material to crack as a result of the weld. A root pass region of the weld is formed using a filler material that exhibits ductility that is higher than that of the base superalloy material. The ductile material mitigates stress in the root region, thereby preventing the formation of cracks. A remaining portion of the weld is formed using a filler material that essentially matches the base superalloy material. However, the amount of “even-matched” filler material build-up is limited to a certain height in order to avoid cracking.
For laser weld repair by deposition there are many different superalloy compositions designed for different applications, and alloy powders and/or alloy wires can be used as the filler materials for component repairs that require considerable strength. However, commercially available superalloy powders or wires are limited in terms of composition and availability. This situation thus largely limits the application of laser welding repair. For example, as noted above, laser depositions at room temperature using INCONEL® 738 powder or wire for repairing INCONEL® 738 blade (i.e. even-matched), or using Mar-M-247®, manufactured by MetalTek, powder or wire for repairing CM-247 blade (even-matched), will in most cases produce significant cracking due to the high contents of Al and/or Ti in the fillers. Therefore, a superalloy composition with reduced Al and Ti contents should be used for repair of INCONEL® 738 and CM-247, preferable at room temperature, for component repairs that require considerable strength. It has been reported that INCONEL® 939, manufactured by Special Metals Company, which contains a much smaller amount of Al compared to INCONEL® 738, could be successfully used in laser welding repair of INCONEL® 738. However, because of the much lower Al content, the resulting weld has a reduced oxidation and corrosion resistance. Further, INCONEL® 939 powder is much more expensive than INCONEL® 738 powder, and is not a commercially available superalloy powder.
U.S. Pat. No. 6,872,912, incorporated herein by reference, discloses an ambient temperature process for almost crack-free welding of a nickel-based single crystal superalloy component containing at least 5 weight percent total of Ti and Al, using a filler alloy which is a composition of nickel based single crystal superalloy, except that the total of Al and Ti is reduced below 5 weight percent. Welds created with this uniformly composed filler material reduce the susceptibility of the weld build-up to cracks even for components that require considerable strength. Those filler alloys, however, are not usually commercially available, and hence are costly and the process is not easy to implement.